The term halosilane as used in the present specification includes any one or more of the following: SiX.sub.4, HSiX.sub.3, H.sub.2 SiX.sub.2 and H.sub.3 Six, and is represented by the general chemical formula H.sub.n SiX.sub.(4-n) where X represents Cl, Br or I.
Recent developments in the semiconductor industry have created a growing demand for low cost single crystal silicon of extremely high purity, which is known as semiconductor grade silicon. Semiconductor grade silicon is used in the manufacture of semiconductor devices, such as transistors, rectifiers, solar cells and the like. Processes are in use in the prior art for the production of polycrystalline semiconductor grade silicon, which can be converted into single crystal semiconductor grade silicon by means of special techniques, such as by the well known Czochralski method.
In one such prior art process, for example, silicon tetraiodide is purified by crystallization, and vaporized, the vapor being subsequently caused to deposit silicon on a hot wire of a relative inert metal, such as tungsten. In such a prior art process, because of the difference in volatility of silicon and iodine, the reaction product iodine vapor diffuses away from the space near the heated wire, and the silicon is deposited on the heated wire and grows to form a substantial silicon crystalline mass. When the growth reaches a certain stage, the cooled mass of crystalline silicon is cut in layers from the wire substrate.
Another prior art process for the preparation of polycrystalline semiconductor grade silicon includes the reaction of super-heated silicon tetrachloride of high purity with highly heated vapor of zinc, causing an interaction of the zinc vapor and the silicon tetrachloride. A heated silicon substrate is provided, and the zinc vapor and silicon tetrachloride cause elemental silicon to grow on the heated silicon substrate to provide polycrystalline elemental silicon which, under suitable conditions, is at least partially of simiconductor grade.
Semiconductor grade polycrystalline silicon has also been produced in the prior art by the reduction of silicon halides with hydrogen in a furnace, the mixture being passed slowly through a heated tube of fused quartz located within the furnace. The silicon is deposited on the inner surface of the heated tube, and the tube is removed from the furnace from time-to-time to recover the silicon.
Semiconductor grade polycrystalline silicon is presently being produced by a chemical vapor deposition process by which trichlorosilane (SiHCl.sub.2), or silicon tetrachloride (SiCl.sub.4), is reduced with hydrogen on a hot silicon substrate at approximately 1200.degree. C, according to the teachings of U.S. Pat. Nos. 3,053,638 and 3,240,623. The trichlorosilane and silicon tetrachloride are prepared in the processes from commercial or metallurgical grade silicon of the order of 98% purity, and they are purified by fractional distillation.
The prior art processes have demonstrated the technical and economic feasibility of producing high purity polycrystalline silicon of semiconductor quality by hydrogen reduction of and halosilanes. All commercial semiconductor grade polycrystalline silicon is presently being manufactured in accordance with the aforesaid chemical vapor deposition process, which employs hydrogen reduction of dichlorosilane or trichlorosilane and the deposition of silicon on an electrically heated silicon filament substrate. The silicon filament substrate is maintained at temperatures above 1000.degree. C by electrical resistance heating, and the walls of the chamber enclosing the filament and reacting gases are maintained at temperatures of the order of 300.degree. C to avoid the deposition of silicon thereon. The heated substrate increases in diameter as the process proceeds until it reaches a diameter of the order of 3 inches to 4 inches. The process is then discontinued until the substrate, which can be up to 4 feet in length, is removed from the chamber and replaced with a new starting rod which, for example, may be of 1/8 - 1/2 inch in diameter. Generally, the continuous vapor deposition reactor effluent gases are not recycled in the prior art process but are disposed by appropriate means.
Large amounts of electrical energy are required to operate the prior art continuous vapor deposition process of the order of 800-1000 kilowatt hours per kilogram of silicon produced. Capital and labor costs are also high due to the multiplicity of reaction chambers and silicon substrates required. The cost of production in a plant producing at a rate of 300-500 metric tons per year is in the range of $25.00-$30.00 per kilogram at the present time. The present-day market price is about $65.00 per kilogram.
The supply and demand for and of the semiconductor industry are in balance at the present time. There have been periods of severe shortages in recent years, and a potentially large new demand which could exceed the present semiconductor industry demand many times over is developing. The new demand is being created by the use of silicon solar cells for the photovoltaic conversion of solar energy into electrical energy. In order to realize this potentially new demand and to supply the demand, it will be necessary to reduce the manufacturing costs of semiconductor grade silicon to substantially less than $10.00 per kilogram, and to maintain silicon quality which will provide high efficiency of conversion of solar energy into electrical energy.
It is among the objects of the present invention to provide a process and apparatus to meet the aforesaid demands and criteria. The present invention provides a process which operates continuously; in which energy requirements are greatly reduced; and in which reactor effluent gases are recovered, separated and recycled. In particular, the hydrogen halide by-product of the process of the invention is recycled to generate purified halosilane feed stock for the reduction reactor, and hydrogen is also recovered and recycled. The only raw material consumed in the process of the invention is low cost metallurgical grade silicon.
The invention also provides a simple continuous flow reduction reactor in which the reactants have a short residence time of the order of 0.01-0.1 seconds. The reactants and nucleants are separately pre-heated in efficient radiant gas fired heat exchangers. The product is granular in the range of 10-100 mesh. The net result is a small tubular reactor with high volume capacity, continuous operation and continuous removal of the product, and representing much lower capital and operating costs than for the prior art continuous vapor deposition processes.
The continuous flow reduction process of the present invention represents a relatively low cost, high volume means for the continuous production of semiconductor grade silicon. In an embodiment to be described, impure (metallurgical grade) silicon is converted into a volatile halosilane intermediate compound according to the following chemical reactions (1) or (3): ##STR1##
Impurities in the form of halides are separated from the halosilane intermediate in the process of the invention by fractional distillation and rejected. The purified volatile halosilane is then pre-heated and reduced in accordance with the chemical reactions (2) and (4) with purified pre-heated hydrogen in a continuous flow tubular reactor. Purified semiconductor grade polycrystalline silicon is separated and recovered from the reaction gas-solid stream. Reaction product hydrogen halide is separated from the reaction gas stream and recycled to convert more metallurgical grade silicon into crude intermediate silicon compounds and hydrogen. Thus the process of the invention consumes impure silicon, and it produces pure silicon, rejecting impurities as liquid or solid halides.